Trimmed Mean Calculator
The mean after discarding a percentage from each end — the compromise between the mean's efficiency and the median's robustness.
A k% trimmed mean sorts the data, removes the lowest k% and the highest k%, and averages what remains.
How the trimmed mean calculator works
A k% trimmed mean sorts the data, removes the lowest k% and the highest k%, and averages what remains. At 0% it is the plain mean; as the trim approaches 50% it converges on the median.
A 10% or 20% trim keeps most of the mean's statistical efficiency while removing its vulnerability to a handful of extreme values. Olympic judging uses exactly this idea.
Formula: trim ⌊kn⌋ from each end, then average the rest
Worked examples
| Inputs | Trimmed mean | Note |
|---|---|---|
| 10% trim on nine values with one outlier | 10.222222 | the 45 is dropped |
| No trim | 10.222222 | equals the plain mean |
| A 40% trim | 6.333333 | close to the median |
FAQFrequently asked questions
What is a trimmed mean?
The mean of the data after discarding a fixed percentage from each end.
How much should I trim?
10% or 20% is conventional. It removes most of the outlier sensitivity while keeping far more information than the median does.
Where is it used in practice?
Olympic scoring drops the highest and lowest judges; the US core inflation measure uses a trimmed mean of price changes; robust regression uses the same idea.
What is a winsorised mean?
Instead of dropping the extremes, it replaces them with the nearest kept value. It uses every observation but caps their influence.
Is trimming cheating?
No, provided you declare it. Choosing the trim after seeing which answer you prefer is cheating; setting it in advance is standard robust practice.
Where these figures come from
- NIST Digital Library of Mathematical Functions — reference definitions for elementary and special functions
- Wolfram MathWorld — definitions and formulas for every topic on this page
- NIST/SEMATECH e-Handbook of Statistical Methods — the statistical formulas (mean, variance, z, confidence intervals, sample size)
- National curriculum in England — Mathematics — the terms and methods taught in UK schools
Last checked: September 2026. Formulas are fixed by mathematics and do not change with tax years or regulations.